I have a six month America visa in my British passport.?
Technically America is the collective term for the continents of North and South America. As such, Canada is part of America. However, since America doesnt issue visas (e.g. like the EU Schengen visa) lets presume for a minute that you mean the USA. If you currently have a valid visa for the USA then yes, you can present this visa at a land border from Canada into the United States and request permission to enter on this visa (note: no visa grants you the automatic right to enter the country - it is up to immigration officials to decide this at the point of inspection). Note: I am not a lawyer and this does not constitute legal advice.